Method and apparatus for performing prehydrolysis in a continuous counterflow digester



April 30, 1968 1. c. F. c. RICHTER ETAL 3,380,833

METHOD AND APPARATUS FOR PERFORMING PREHYDROLYSIS IN A CONTINUOUS COUNTERFLOW DGESTER Filed June 5. 1964 United States Patent O 3,380,883 METHOD AND APPARATUS FOR PERFORMING PREHYDROLYSIS IN A CONTINUOUS COUN- TERFLOW DIGESTER Johan C. F. C. Richter, St. Jean Cap Ferrat, Alpes-Maritimes, France, and Ole J. Richter, Karlstad, Sweden, assiguors to Aktiebolaget Kamyr, Karlstad, Sweden, a company of Sweden Filed June 5, 1964, Ser. No. 372,826 Claims priority, application Sweden, Nov. 27, 1963, 13,093/ 63 3 Claims. (Cl. 162-19) ABSTRACT F THE DISCLOSURE A digester arranged for downwardly directed feed of the treated cellulosic material is provided wherein it is elfectively possible to avoid the reaction interaction difficulty when the digestion therein is performed with the digesting liquor driven in counterilow to the ow of cellulosic material, i.e. upwardly in the digester. An essential characterizing feature of the invention resides in the maintaining of a free liquid level somewhat above lthe point where the digesting liquor used for the digestion is screened olf the digester and additionally in that the prehydrolysis is performed in steam phase above said free level. Due to the existence of said free liquid level it is much easier to keep the prehydrolysis and the digestion apart than if e.g. the prehydrolysis would take place in liquid phase, in which case the liquids could mix and a definite border layer hardly could be maintained. When according to one form of the invention the prehydrolysis in steam phase is followed by a continued prehydrolysis in liquid phase, i.e. below said free liquid level, at least the iirst step of the prehydrolysis will be efciently separated from the digesting liquors.

The invention relates to a method and apparatus for the performance of prehydrolysis in an elongated upright container arranged for counterflow digesting cook of cellulosic material, such as wood chips fed continuously and downwardly through the digester.

Prehydrolysis as a preliminary step to a digesting cook has according to the prior art ybeen performed in a separate container arranged for upwardly directed feed of the cellulosic material, for instance as shown in the Swedish Patent No. 122,608. In order to simplify and lessen the expense of pulp production it is a desideration to perform the prehydrolysis in the same vessel as is used for the digesting cook. However, the realization of such an improvement has not heretofore been accomplished because of the difliculty of keeping the hydrolysis zone and the digesting zone apart, this being essential from a practical standpoint in order to have the two reactions take place without mutual disturbances and interactions. It is therefore a primary object of the present invention to provide a method and apparatus for continuously performing prehydrolysis and digestion of cellulosic material such as wood pulp in effectively isolated reaction zones within the same vessel.

According to the present invention, a digester arranged for downwardly directed feed of the treated cellulosic material is provided wherein it is effectively possible to avoid the reaction interaction diiculty when the digestion therein is performed with the digesting liquor driven in countertiow to the flow of cellulosic material, i.e. upwardly in the digester. An essential characterizing feature of the invention resides in the maintaining of a free liquid level somewhat above the point where the digesting liquor used for the digestion is screened off the digester Mice and additionally in that the prehydrolysis is performed in steam phase above said free level. Due to the existence of said free liquid level it is much easier to keep the prehydrolysis and the digestion apart than if e.g. the prehydrolysis would take place in liquid phase, in which case the liquids could mix and a definite border layer hardly could be maintained. When according to one form of the invention the prehydrolysis in steam phase is followed by a continued prehydrolysis in liquid phase, i.e. below said free liquid level, at least the tirst step of the prehydrolysis will be efficiently separated from the digesting liquors.

The plant for carrying out the novel method includes means for effecting prehydrolysis in an upright cellulose digester of the kind which is provided at its upper end with continuously operative charging means for cellulosic material, such as Wood chips, and is provided at its lower end with continuously operative discharging means for digested pulp. The digester is further provided at a center zone with sieves for screening olf and discharging such digesting liquor as has been supplied at a lower level of the digester and which has passed up through the digester in counterow to the cellulosic material while digesting the same. The plant according to the invention importantly includes a steam conduit connected to the upper end of the digester for direct supply of steam and maintenance of a steam atmosphere therein. Also according to the invention a level control device is arranged to maintain a, free liquid level in the digester above the level of said sieves.

The objects just mentioned as Well as other objects of the invention will become more apparent during the course of the following detailed explanation wherein particular reference is made to the attached drawing in which apparatus for carrying out the process of the invention and embodying the principles thereof is illustrated. The apparatus shown is to be understood in this illustrative sense rather than as limiting the scope of the invention.

-ln the drawing:

The ligure is a generally schematic flow diagram illustrating novel apparatus for carrying out the method of the invention.

In the drawing, 11 designates a standing cylindrical pressure-resistant vessel of a length preferably approximating 10 times its diameter. The vessel 11 is provided at its upper end with charging means for continuous charging of chips coming from a chip bin `15 provided with a rotatable measuring Wheel 13. The charging means preferably consists of a rotor 17 inserted in a casing 16 and having a diametrically extending pocket P which alternatingly is filled with chips from an inlet upon the casing and is emptied through an outlet thereon by means of a steam conduit 19 connected to the casing. When the pocket takes its emptying position, steam ows from the conduit 19 into the vessel 11 forcing the chips in the rotor pocket P with it and thus into the vessel 11. rl"he steam conduit 19 could ybe connected directly to the shell of the vessel 11. The details of the charging means may be the same as are shown in Swedish Paten-t No. 165,686.

The chips W charged into the vessel 11 do not fill the vessel completely but merely up to a level 21 which is ascertained by a level indicating means 23 such as a photocell system. The level 21 is preferably maintained essentially constant by controlling the charging rate of the chips. ln the zone a in the upper portion of the vessel 11 as shown in the drawing, where the spaces between the chip particles are only lled with the steam which has entered through conduit 19, the chips are heated to a temperature of approximately to 170 C., by the steam. The wood is hydrolyzed in this zone by means of acids naturally present in the wood or additionally by acids such as sulphuric acid in which the chips may be presoaked. Condensate and hydrolysate obtained by the hydrolyzation reaction flows down through the chips column and merges with the liquid filling the Vessel in the zone b, below the level 25, which level is maintained in a manner to be described below.

IBelow the zone a there is a zone b where the hydrolysis continues but now in liquid phase. A digesting cook takes place in the next zone c, and washing takes place in the lowermost zone d. At the lower end of the digester, continuously operating discharging means 27, of conventional design, operate to discharge the digested pulp continuosly, so that the pulp contained within the vessel 11 slowly moves downwardly, thereby succesively passing through the above-mentioned various zones. The pressure of the steam supplied through the conduit 19 is preferably of the order of l atmospheres and may be adjusted to maintain the desired super-atmospheric pressure in the vessel.

At the lower end of the digester a conduit 29 supplies washing liquid which may consist of water, diluted spent liquor or the like. The washing liquid is forced upwardly in counterflow to the pulp, thereby washing out digesting liquor from the downwardly moving pulp column. -Part of the washing liquid is sieved off through a sieve girdle 31 and freshA digesting liquor, such as sulphate digesting liquor, from the conduit 33 is supplied to an outer circulation loop in which also heating by means of a heat exchanger 35 is performed. The liquid is circulated by means of a pump 37 and is supplied through a conduit 39 to a point located on the axis of the digester and somewhat above the level of the sieve girdle 31. The heated digesting liquor thus supplied is `forced in counteriiow to the cellulosic material up through the digesting cook zone c of the vessel. A sieve girdle 41 is inserted in the shell of the vessel at a distance from its upper end approximately equal to a third of the length of the vessel. The spent digesting liquor is sieved off the downwardly passing chips column through said girdle 41 so that above the level of said sieve girdle 41 there is no motion of liquid in the upward direction.

-Immediately above the sieve girdle 41 there is inserted in the shell of the vessel a further sieve girdle 43. The above-mentioned condensate and hydrolysate are extracted through said sieve girdle 43. Due to the flow of said liquids in the zone b being opposite to the direction of flow of the digesting liquor, the tendency of the latter to extend upwardly into zone b, due to convection is effectively prevented. Also, the hydrolysate is prevented from extending downwardly into the zone c, inasmuch as it would there meet a heavy upflow of spent liquor and be displaced thereby. It has been found that a small amount of hydrolysate accompanies the hydrolyzed wood chips by being enclosed in the pores thereof, but this hydrolysate lgradually reaches the surface of the chip particles as they move downwardly and is there picked up by the counterow of digesting liquor, so that even these minor quantities of hydrolysate are removed and prevented from interfering with the digesting cook taking place in the zone c.

The sieve girdles 41 and 43 are provided with separate outlet conduits 47 and 49, respectively, in which valves may be inserted, making it possible to adjust the position of the border surface between upwardly and downwardly flowing liquids in the digester. In instances where this interface adjustment is not considered sufficiently imporant the two sieve girdles may be combined into a single one.

In order to maintain the free liquid level at a desired position there is provided a level controlling device shown consisting of a separate Vessel 51 which at its lower end communicates with the liquid contents of the digester through the conduits 47, 49 and its upper end communicates through the conduit 53 with the steam atmosphere of said vessel 11. Therefore, in the vessel 51 the liquid 21. 1 level 55 takes a position corresponding to the level 2 in the digester.

The lower end of the -vessel 51 is connected by means of a conduit 57 to a blow tank 59 in which a pressure is maintained which is considerably lower than the pressure within the digester. Inserted into the conduit 57 is a choke valve 61 controlled by a level sensing device 63 such as an adjustable float. The valve 61 admits spent digesting liquor from zone c mixed with hydrolysate from zone b to the blow tank 59 at such a rate that the level 55 and therefore the level 25 in the digester are maintained at the adjusted desired position. Should it be desired to adjust the ratio between the part of the hydrolysis performed in the liquid phase and the part performed in the steam phase, this is possible within certain limits by adjusting the level sensing device 63. From the blow tank the digesting liquor is discharged through a bottom outlet and transferred to a recovery plant, whereas the steam obtained in the blow tank due to the pressure drop therein is discharged through a conduit 65.

Air which may have been introduced into the digester, for instance within the wood, and non-condensible gases formed therein are exhausted from the system through the conduit 67 leading from the upper end of level controlling vessel 51.

The cellulosic pulp which has been treated by prehydrolysis, digestion and wash, leaves the vessel 11 through a conduit 69 connected to discharging means 27 and is concentrated in a press 71. The pulp then flows ou through a conduit 73 for continued treating.

It should now be apparent from the foregoing description that a novel method and novel apparatus have been set forth which accomplish the objects of the present invention as delineated hereinabove.

Inasmuch as the method and apparatus embodiments shown and described herein allow the principles of the present invention to be more clearly comprehended and are not intended to limit the scope of the invention as it may be modified while adhering to said principles, the invention should be limited only by the spirit and scope of the following claims.

We claim:

1. A method for prehydrolyzing cellulosic material in a digester comprising: continuously feeding digesting liquor-free cellulosic material into said digester near the upper end thereof; continuously moving the cellulosic material downwardly through the digester; introducing hot liquid digesting liquor into the vdigester in the lower part thereof, digesting the cellulosic material with said liquor and withdrawing spent liquor from the digester at a point intermediate the upper and lower ends of the digester; maintaining the upper level of liquid digesting liquor a short distance above the liquor withdrawing point by controlling the rates of liquor introduction and withdrawal; introducing steam into the digester above the level of said liquor and at least partly prehydrolyzing the cellulosic material before moving said cellulosic material downwardly into the digesting liquor; and removing the prehydrolyzed and digested cellulosic material from the digester near the lower end thereof.

2. A method as set forth in claim 1 wherein the upper level of liquor is adjusted to proportionate the amounts of prehydrolysis performed in the gas phase, above the upper level of the liquor and in the liquid phase, between the upper level of the liquor and the liquor withdrawing point.

3. Apparatus for prehydrolyzing and digesting cellulosic material including: a digester having an upright, elongated chamber therein, continuously operative pulp charging means communicated to the chamber near the upper end thereof; continuously operative pulp discharging means communicated to the chamber near the lower end thereof; a digesting cook zone in the chamber intermediate the upper and lower ends thereof; first means communicated to said zone at the upper extent thereof operative to separate and remove iiuids from the chamber; second means communicated to said zone at the lower extent thereof operative to separate and remove fluids from the chamber; means for adding steam to the chamber above said rst iluid separating and removing means; means for adding liquid digesting liquor to the chamber Within said zone near the lower end thereof; and control means operative to maintain the upper level of said liquid digesting liquor above the level of said lirst fluid separating and removing means; the liquid digesting liquor level controlling means including: a separate vessel so positioned as to extend above and below the desired level of liquid digesting liquor in said digester, a first conduit connecting the lower end of the separate vessel with said iirst fluid separating and removing means for transmission of separated and removed liquid digesting liquor to said separate vessel, a second conduit connecting the separate vessel with said chamber above said lirst iiuid separating and removing means and Communicating the separate vessel with the steam atmosphere of said chamber, whereby a liquid level is maintained in said separate vessel corresponding to the level of liquid digesting liquor Within said digester, an outlet conduit connected to said separate vessel below the iiquid level therein, an adjustable choke valve interposed in said outlet conduit, a level sensing device in said separate vessel, said level sensing device being constructed and arranged to sense the level of liquid in said separate vessel, and means operatively connecting the level sensing device to the choke valve for regulating said choke valve in response to sensation of the level of liquid in said separate vessel.

References Cited DONALL H. SYLVESTER, Primary Exmzilze'.

o HWARD R. CANE, Examiner. 

